It is known in the art to manufacture organic isocyanates by reacting organic nitro compounds with carbon monoxide in the presence of a catalyst in a liquid phase.
British Pat. No. 1,025,436 discloses a process for preparing isocyanates from the corresponding nitro compounds by reacting an aromatic nitro compound with carbon monoxide in the presence of a noble metal based catalyst in a liquid phase. It has been reported, however, that this process is not used commercially because no more than trace amounts of aromatic isocyanates are formed when an aromatic nitro compound such as dinitrotoluene is reacted with carbon monoxide using a noble metal based catalyst such as rhodium trichloride, palladium trichloride, iridium trichloride, osmium chloride and the like.
For years, attempts have been made to find more efficient methods for producing organic isocyanates by the catalyzed carbonylation of nitro compounds at elevated temperatures and pressures. For example, U.S. Pat. No. 3,576,836 to Prichard teaches a process for preparing aromatic isocyanates by carbonylation, at elevated temperatures and pressures, of aromatic mono- or dinitro compounds in the presence of a palladous halide and an organic cyano compound. U.S. Pat. No. 3,728,370 to Ottman et al. teaches a process for providing organic isocyanate by reacting an organic nitro compound with carbon monoxide in the presence of a noble metal based catalyst and an unsaturated organic compound having at least one multiple bond conjugated to another multiple bond in selected aliphatic or cycloaliphatic systems or in the presence of an unsaturated organic compound having at least one multiple bond conjugated to an aromatic hydrocarbon nucleus.
Other catalysts taught in the prior art are a mixture of a noble metal based catalyst and a non-noble based catalyst, U.S. Pat. No. 3,523,966 to Ottman et al.; a mixture of a noble metal based compound and an organophosphorus compound, U.S. Pat. No. 3,523,962 to Kober et al.; a mixture of (a) an inorganic copper compound, (b) at least one compound selected from the group consisting of elemental palladium, elemental rhodium, palladium halides, rhodium halides, palladium oxides and rhodium oxides, and (c) at least one oxide of an element selected from the group consisting of vanadium, molybdenum, tungsten, niobium, chromium and tantalum.
Further, U.S. Pat. No. 3,754,014 to Kober et al. discloses carrying out the carbonylation process in the vapor phase in the presence of a noble metal based catalyst and preferably utilizing a halide of copper as a catalyst component.
Quite surprisingly, it has now been discovered that high yields of aromatic isocyanates and diisocyanates can be derived from aromatic nitro compounds simply by reacting an aromatic dinitro compound with carbon monoxide at an elevated temperature and an elevated pressure in the presence of a rhodium oxide catalyst and an organic nitrile compound.